Optical Encoders have been widely used as position feedback devices in systems for controlling the position of the rotary shaft in different kinds of rotary devices. Thus for example Optical Encoders are widely used in Robots, automatic machines and similar devices.
Examples of such optical encoders can be found in patents U.S. Pat. No. 4,268,747 by Becchi et al. (1981), and U.S. Pat. No. 4,410,798 by Breslow (1983).
Optical encoder systems generally include a rotating optical disc, with a pattern of sections of alternating optical properties (for example transparent and opaque sections) fixed on the motor shaft. The disc is placed in the path of an optical beam, between a light emitter and a light sensor. The light sensor then creates an electric signal with amplitude changing periodically with the shaft position. An electronic circuit is used to count the number periods of that electronic signal, and thus provide information on the shaft position, relative to an initial position.
In order to sense rotation direction, in most of the prior art optical encoders two optical beams are used, and the path of the second beam is positioned so as to produce a second electrical signal similar to the first one, but shifted by a quarter of the period of the first signal. Such two signals will be further referred to as being in quadrature.
An improvement of this method consists of using a light beam that covers several segments of a circular pattern section with alternating optical properties provided on a rotary disc and adding a fixed mask that has a pattern similar to the rotating optical disc pattern, containing several consecutive segments with alternating optical properties and covering at least the surface of the light sensor between the rotating optical disc and the light sensor. In this arrangement, when mask and disc patterns coincide, a maximum of light is transmitted whereas when mask and disc patterns are in opposite phase, light transmission is minimum. Thus, the light transmitted to the sensor becomes a periodic function of the angular position. This method provides an improved signal shape, due to the fact that tolerances in the exact shape of pattern segments are averaged over a number of segments.
It is also known to enhance encoder resolution by feeding the analog amplitude of the signals in quadrature to a processing unit. The processing unit can then be programmed to calculate the angle of the shaft within a single segment of the pattern, thus providing a much higher resolution. For example a disc with 512 slots will provide an angle resolution of 1/65536 of a cycle.
There is a continuing trend of improving the precision and resolution of the position feedback devices, while reducing costs.
One factor limiting precision is the slightly eccentric movement of the rotating optical disc, which is due to some mechanical tolerance. Since each of the two beams intersects the optical disc at a defined position on the disc, on one side of the shaft, any lateral movement of the shaft will affect the amount of light that reaches the light detector, thus creating error in the position information. Such a lateral movement may be caused by tolerance in the roll bearing holding the shaft, or by tolerance in the optical disc assembly. In order to avoid this kind of error, many encoders include an integral shaft and high precision bearing and are coupled to the motor shaft by means of a coupling whereby costs are considerably raised.
Another factor is the precision of the pattern on the rotating disc. Irregularities in the pattern generate unequal periods relative to the angular position of the shaft. Where the two beams intersect the optical disc at respective defined positions on the disc on one side of the shaft, as described above, irregularities of the disc pattern will influence the amount of light that reaches the light detector, and again introduce error into the position information.
In the Optical Encoder of PCT/IL 2004/000042 improved precision is obtained by using two or more light guides that project the light on the rotating disc in the form of two conical beams that create two concentric circles of light having a geometrical center that coincides with the rotation symmetry center. On the optical disc a pair of concentric annular pattern sections, each having a large number of segments with alternating optical properties are provided, the geometrical center of the annular patterns coinciding with the rotation symmetry center, and the diameters of the said concentric circles of light being chosen such that each of the concentric circles of light is incident on one of the concentric annular patterns and covers the said annular pattern. This radial symmetry of the inventive Optical Encoder provides compensation for small lateral movements of the shaft and since light signals are collected from a large number of segments equally distributed around the center of symmetry, the effect of irregularities in the shape of the segments is averaged and thus reduced. In the Optical encoder of PCT/IL 2004/000042 both the light that is emanated from the light source towards the optical disc and the light that returns from the optical disc to the sensors are conducted through optical fibers that enter the light guide at an entrance surface. The optical encoder being distant from the electronic board that carries the sensors and from the light source, one or more optical cables are required for this system in addition to a standard electronic cable.
Due to the separation and distance between the optical disc, the light source and the electronic board in prior art Optical encoders, design and wiring problems arise concerning the mounting of these separate parts between the components of the rotating device while at the same time production and mounting costs are increased.
None of the prior art Optical encoders propose an integral electronic board that is located within the Optical encoder assembly and carries the light source, the light sensors and the signal processing means directly on a PCB such that both light source and sensors are close to the symmetry axis of the rotating device, thereby further increasing precision and reducing manufacturing and mounting costs.
An object of the invention is to provide an Optical encoder for a rotary shaft that increases precision and reduces wiring, production and mounting costs by a new integral and symmetric design in which light sensors and light source means are installed on each of the faces of a single PCB and the PCB is placed between a pair of light guides that receive light emitted from opposite sides of the PCB, guide it onto an optical assembly that is suitable for indicating the rotation angle and finally guide the returning light towards the light sensor means on the PCB.